Threshold switch and novel material therefor

ABSTRACT

To form a powder that exhibits a change in electric conductivity in response to an applied field, a plurality of minute particles of conductive material are each coated with an amorphous semiconducting material. As used in a threshold switch system, the powder is sandwiched between a plurality of electrodes. The electrodes are continuous or distributed, depending upon the species. Distributed electrodes also find utility with continuous layers of such amorphous semiconducting material. Advantageously, such switches are used in image display panels.

United States Paten Sohel 1 1 Oct. 23, 1973 [5 THRESHOLD SWITCH AND NOVEL 2,150,167 3 1939 Hutchins et al 338/21 x MATERIAL THEREFOR 2,067,393 1/1937 Habann 338/223 X [75] inventor: Alan Sobel, Evanston, 111. Primary Examiner C- L Albritton [73] Assignees Zenith Radio Corporation, Chicago, t ney-John H. Coult et al.

22 Filed: Apr. 10, 1972 [57] ABSTRACT To form a powder that exhibits a change in electric [21] PP 242546 conductivity in response to an applied field, a plurality of minute particles of conductive material are each [52] U.S. Cl 338/32 R, 117/212, 338/223, ed w an amorphous semiconducting material. 338/225, 340/166 EL As used in a threshold switch system, the powder is [51] Int. Cl 1-101c 7/16 n wiched between a plurality of electrodes. The [58] Field of Search 338/32 R, 223, 225, electr are continuous or distributed, depending 338/20-24; 340/166 LL; 117/212; 106/47 R upon the species. Distributed electrodes also find utility with continuous layers of such amorphous semi- [56] Referen e Cited conducting material. Advantageously, such switches UNITED STATES PATENTS are used in image display panels.

2,329,085 9/1943 Ridgway 338/21 X 11 Claims, 11 Drawing Figures Line Sconnen Element Scanner PAIiNIEunmza 197s 3,7683% FIG. 1

Video Sync: Element Scanner Line Scanner FIG. 3c 2'] X 24 21 THRESHOLD SWITCH AND NOVEL MATERIAL THEREFOR BACKGROUND OF THE INVENTION The present invention pertains to electric switch systems. It also relates to switch systems useful in image display panels.

Solid-state light generation or light control devices have been suggested for use in flat image display apparatus. Thus, solid-state diodes, electroluminescent cells, liquid crystals and mechanical shutters have been distributed over display matrices and selectively activated individually in order to create the display of an image. Many of these approaches involve the use of a multiplicity of switches also distributed over the face of the display panel and require that each switch exhibit a threshold effect.

In US. Letters Pat. No. 3,647,958, issued Mar. 7, 1972, and assigned to the same assignee as the present application, for example, there is described and claimed an approach which increases available contrast by including a plurality of minute switches at each image point. In this as well as in various other display panels, one difficulty which may be encountered is image distortion resulting from the failure of a comparatively few of the perhaps many-thousand switches present in a single panel. It also is apparent that the fabrication of a device employing such a large number of switches poses difficult manufacturing problems.

One form of switch suggested for use in the aforesaid patent utilizes as its active element an amorphous semiconductor material. Placed between conductive electrodes, the material exhibits the desired threshold characteristic. While capable of being formed to have the minute size needed in an image display panel intended, for example, to be used in a television receiver, at least many of the possible difficulties adverted to above still remain. Similar or analogous problems may be found in other and different applications that require the use of threshold switches.

It is a general object of the present invention to provide a new and improved threshold switch system which overcomes the aforenoted difficulties of prior switch systems.

Another object of the present invention is to provide a new and improved threshold switch system that permits the attainment of an increase in production yield of devices in which the switches are used I BRIEF DESCRIPTION OF THE DRAWINGS The features of this invention which are believed to be novel are set forth with particularlity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in conjunction with the accompanying drawings, in the several figures of which like reference numerals identify like elements, and in which:

FIG. 1 is a diagram of an image-display panel together with an addressing system;

FIG. 2 is a fragmentary perspective view of one embodiment of an image-display panel;

FIG. 3a is a fragmentary cross-sectional view taken along the line 3-3 in FIG. 2;

FIG. 3b is an electric field plot useful in explaining the operation of the embodiment of FIG. 3a;

FIGS. 3c and 3d are fragmentary cross-sectional views depicting alternative details in FIG. 3a;

FIGS. 4 and 5 are fragmentary cross-sectional views of switch systems which may be utilized in the embodiment of FIG. 2;

FIG. 6 is a cross-sectional view of a preferred form of switch element; and

FIGS. 7 and 8 are fragmentary cross-sectional views of respective different switch systems incorporating the switch element of FIG. 6.

While the switch systems to be described find utility in a variety of applications, they are particularly attractive as incorporated into image display panels. FIGS. 1 and 2, therefore, serve to illustrate one particular display panel in which the switch systems advantageously are used. However, the switch systems also are desirable for employment in numerous other forms of display panels.

In FIG. 1, an image display panel has a plurality of elongated conductors or column electrodes 10 laterally spaced apart across one surface of the panel. On the rear side of the panel (not shown) are another series of elongated conductive elements or row electrodes also laterally spaced apart across the panel but oriented orthogonally to column electrodes 10. An element scanner ll selectively addresses different ones of column electrodes 10 with individual control signals. In this case, those control signals include an input signal component that is proportional in amplitude to respective different levels of picture information derived from a source 12 of video. At the same time, line scanner 13 addresses different ones of the row electrodes with enabling signals.

Scanner 11 responds to column-selection signals from a synchronizer 14 which also supplies rowselecting signals to scanner 13. The selection of any one row conjointly with the selection of a respective column addresses the intersection of that row and column. Assuming a light-display element to be located at that intersection, its level of energization depends, in this version, upon the amplitude of the control signal applied to that column, and that amplitude, in turn, corresponds to the level of the picture information from video source 12. Prior panels of this kind are known wherein the panel itself includes a layer of electroluminescent material disposed between the mutually-crossed arrays of conductors. Each intersection, where one conductor spatially crosses another, defines the location of a picture element.

Scanners 1'1 and 13 may take any of a number of known forms. One conventional approach is to include in each scanner a shift register that is stepped from each one output to the next by a series of gating pulses in turn initiated by a timing clock that is synchronized with the signals from synchronizer 14. For use as a television display, scanner 13 selects rows sequentially in succession from top to bottom and, while each such row is selected, scanner 11 sequentially selects successive ones of columns 10 from left to right. Upon conclusion of one complete scan of all elements in the panel, the synchronizing information resets the scanners so that the scanning process begins anew. Thus, the image as viewed over a period of time represents a succession of frames within each of which the video information is displayed line-by-line, just as in the conventional technique of image scanning upon the face of a cathode-ray tube. In the form illustrated in FIG. 1, the

video component is applied sequentially across each row. In an alternative and well-known approach, all

columns in a row are addressed simultaneously. To that end, scanner 11 may include a bank of storage elements into which each line of video information is first stored. When a row then is selected, the bank is dumped to distribute the stored video components into all of the respective columns at the same time.

Whatever the form of addressing, there also is flexibility in choosing the nature of the different addressing signals. For example, the signal to the row electrodes might simply be the completion of a ground return, while the entire selection potential and the video components are applied to the column electrodes. On the other hand, there may be mixed selection potentials and video modulation on both rows and columns. For addressing an entire row simultaneously, the video modulation is fed to the columns while the selection potential preferably is divided between the rows and columns. In general, two functions are required to activate any particular picture element. The first is selection, that is, the addressing or selecting of the specific picture element or elements to be acted upon. The selection process requires a well-defined threshold or nonlinearity at each picture element by reason of which potentials below the threshold do not perturb thestate of the element. The second function is modulation, the delivery of assigned signal amplitudes to the selected picture elements in order to produce the desired light outputs. For displays which do not have gradations in output level, selection and modulation may be combined. To reproduce gradations in contrast or color, however, independently controllable modulation is required.

Although it might complicate any particular arrangement of the FIG. I approach, it is to be further noted that the video components may be supplied to each effective picture element by a separate array of conductors or equivalent addressing means. In any event, the addressing may be according to a repetitive program as in television or be selective upon command. Moreover, the matrix pattern of FIG. 1 has been described in terms of its orthogonally-related rows and columns in order to clarify the presentation by reference to such a well-understood form of array. However, such language is intended to embrace such equivalent display patterns as those used in plan-position indicators and similar read-out apparatus.

As shown in FIGS. 2and 3a, an image display panel 20 similarly includes a plurality of conductive column electrodes 21 distributed in one direction across one major surface and a plurality of conductive row electrodes 22 distributed in the orthogonal direction across the opposite major surface of the panel. Sandwiched between the opposing arrays of electrodes are a layer 24 of electrically insulating material, a film 25 of anisotropically-conducting material, a perforated sheet 26 also of electrically insulating material, a slab 27 of electroluminescent material and, finally, a glass substrate 28 to which row electrodes 22 are affixed. Situated within each of the perforations or apertures in sheet 26 is a switch system 30.

A preferred version of one of switch systems 30 is described hereinafter in connection with FIG. 7. Therein, each switch system is composed of a multiplicity of individual switch elements formed as a powder. Other arrangements are described in connection with FIGS. 4,

5 and 8. For the moment, however, each switch system may be thought of as if it were a single, unitary switching element or switch. This latter form also is described in my co-pending application Serial No. 240,060, filed Mar. 31, 1972, and assigned to the same assignee as the present application. Each individual switch, then, is characterized by being normally non-conductive but quickly becoming conductive when subjected to a predetermined electric field strength. When in its conduc: tive state, the field traversing the switch energizes the directly-adjacent portion of electroluminescent slab 27. Thus, the electroluminescent material in itself constitutes an effective plurality of light-display elements, each different one individually being associated with a respective different one of switches 30. Moreover, all of the different light-display elements under the control of a given one of column electrodes 21, in association with a similarly given one of row electrodes 22, together constitute a single light-display device defining a picture element and exhibiting a level of light output which is dependent upon the number of its associated switches 30 that are in a conductive state at any given instant of time.

FIG. 3b exemplifies the manner in which the group of switches 30 is controlled in operation by means of the non-uniform electric field extending between the column electrode 21 and the row electrode 22 which together serve to define the location of a given picture element. With but a comparatively small potential difference between electrodes 21 and 22, a significant electric field appears only more or less directly beneath electrode 21. When that field reaches the switch threshold level, those of the switches closest to electrode 21 are first actuated so as to become conductive. As the potential difference between electrodes 21 and 22 is increased, the field disperses or spreads among switches 30 and ultimately encompasses all of the switches in the group associated with electrode 21 at a strength sufficient to activate the switches. Thus, as the field strength between the intersecting electrodes increases in correspondence with an increased applied video level there is a proportional increase in the number of switches 30 that are rendered conductive. Correspondingly, there is a like increase in the total level of energization of the light display element associated with the group of switches under the influence of electrode 21 in FIG. 3a. In this way, it can be seen that there are a multiplicity of energizing switches for each image point and that the number of switches enabled at any given instant depends upon the applied video level. Moreover, the basic control mechanism is that of degree of dispersal of the non-uniform field which, in turn, is proportional to the video level. In this connection, however, it is to be observed that it is a selection potential which controls the field strength at any' giv'en switch so as to enable it to become conductive. In turn, the modulation potential determines which switches are fired at each picture element position. At the same time, the values should be chosen so that a selection signal on only one of the associated row or column electrodes, together with maximum video modulation, is not productive of a potential above the threshold level of any of the switches in the group at that picture element position.

As indicated, light generation in the panel of FIG. 3a

is obtained by utilizing an electroluminescent material.

Accordingly, row electrodes 22 are transparent. Moreover, the row electrodes may be either on the exterior or interior surface of substrate 28. As is well known, electroluminescent cells generate light when subjected to an electric field that exceeds a predetermined threshold level. Usually, the field is developed by the application of an alternating potential, although sometimes a uni-directional potential or a combination of alternating and unidirectional potentials is employed. Display elements other than electroluminescent cells may be utilized instead. For example, alternative light generators include injection-luminescent diodes and gas-discharge cells. For light modulation instead of generation, suitable alternative elements include orientable suspended particles, liquid crystals and electromechanical shutters. In any case, the particular kind of light-display element employed herein is one which responds to energization from an external source to emit or control light. There may be one integral display device for each group of switches, or the display device may, in effect, be sub-divided as by using a plurality of display elements at each picture element position. In the latter case, there may be one separate display element for each different switch.

Each of switches 30 exhibits a predetermined firing level in response to the applied field. While that firing level as between the different switches in a given group may be the same for all, some variation from switch to switch within a group may be tolerated and even advantageous in tending to wash out otherwise visible graininess or other analogous effects. In any event, the ultimate light output from any given display device is proportional to the number of switches which at any given instant are conductive. At the same time, in the version shown, the total light output or brightness of each light display device is subject to a cumulative effect which increases the contrast ratio to an amount even greater than the number of switches per image point. This occurs'because each of the electroluminescent sub-elements is in itself also voltage dependent. The first one excited produces still more light as the field spread and strength is increased to activate the second one, and so forth.

Each of the switches is physically distinct from its associated light-control device. In the aforementioned copending application, solid-layer ovonic switches, constructed of amorphous semi-conducting glass material, are used. Such switches, as such, are described in a article by George Sideris entitled Transistors Face an Invisible Foe," which appeared in Electronics, pages 191-195, Sept. 19, 1966, and in an article entitled Amorphous-Semi-Conductor Switching by H. K. Hanisch which appeared at pages 30-41 of Scientific American for September, 1969. As there described, each ovonic switch may besimply a small layer or dot of a glass-like material deposited between electrodes. Differences in material constituents or in thickness permit the ovonic switches to exhibit different threshold levels. The threshold voltage apparently is a function of the energy band-gap structure of the material. Further detailed description of the amorphous semi-conductor material itself may be found by reference to U.S. Letters Pat. No. 3,395,446 issued Aug. 6, 1968 in the name of A. Jensen and 3,271,591 issued Aug. 6, 1966 in the name of S. R. Ovshinsky.

Whatever the form of switch selected for use in display panel 20, it is preferable that the switch, either alone or in combination with the parameters of the associated elements, exhibit bistability in the sense that, once fired, each switch continues to pass current from a source of sustaining voltage to the light-control device as long as a certain minimum potential is maintained. Accordingly, a sustaining potential sufficient to energize the light display device may exist continuously across conductors 21 and 22 and be of a value which may be only slightly below that required to develop the minimum field level necessary to fire any of the associated switches. On the other hand, the level of sustain potential required may be substantially less than the potential required to activate the switch initially. Using alternating-current excitation for a breakback type of switch associated with a capacitor, e.g., an ovonic switch material in series with a capacitive electroluminescent cell, the sustain level required may be only a fraction of the control pulse amplitude used to fire the switch. In any event, the desired numbers of switches are then actuated simply by superimposing a control pulse upon the sustaining potential so as to raise the total potential level above the desired threshold level and result in the desired degree of field spread. Ovonic memory switch elements similarly may be employed; as is known, these require the affirmative application of an appropriate turn-off pulse.

DESCRIPTION OF THE PREFERRED EMBODIMENT In operation, then, a sustaining voltage may be maintained throughout each frame interval between all of the leads connecting scanner 11 (FIG. 1) to the columns and the leads connecting scanner 13 to the rows. The addressing systems thus permit continuous device energization following the application of the video component of a control signal. That is, each of the light-display devices that has been actuated, in whole or in part, during the most recent frame interval remains in that state by virtue of the sustaining voltage continued during the same interval. Therefore, each of the display devices exhibits persistence or storage, as a result of which the overall image is substantially brighter than would be the case if light were produced only at the instant of addressing each individual display device. Correspondingly, line scanner 13 may serve the additional function of extinguishing all of the display devices in each row shortly before that row is addressed anew during the succeeding frame. To this end, it is only necessary that a shift register or other rowaddressing device break the connection to each row before that row is again selected.

Insulating layer 24 serves, first of all, to insure against direct conductivity between column electrode 21 and the ones of switches 30 located directly beneath elec- I trode 21.- At the same time, layer 24 provides an increased dielectric constant in the field path; this may be used to improve the transfer characteristic as between light output and input signal. For these purposes, the insulating layer need only be approximately of the same width as that of electrode 21. When layer 24 does not extend over all of the switch systems in a group, a highresistance layer may, in accordance with one alternative, be disposed to overlie the remainder of the group of switches so as to modify the field distribution. On the other hand, and as actually shown in FIG. 3a, insulating layer 24 is extended out over the entire area covering all of switches 30. With this construction, the thickness of insulating layer 24 may be tapered or otherwise varied as desired in order to tailor the overall transfer characteristic toward the ultimate end of obtaining whatever contrast scale is most effective for any particular combination of type of switch and type of light output device.

Anisotropic film 25 is not essential to achievement of the basic principle of distributing switches 30 throughout a non-uniform field and selecting the number of the switches to be activated at any given instant by means of field dispersal. When used, however, film 25 exhibits a finite ratio between its resistance in the plane defined by the film and its resistance in the direction normal to 'thatplane. This feature results in an increase in taper of the field gradient as measured in the direction of distribution of switches 30. Moreover, the resistance that film 25 does present in the direction normal to its plane also guards against catastrophic failure of an entire display device. That is, should any one of switches 30 break down entirely so as to remain conductive regardless of applied field level, the included series resistance presented by film 25 may be utilized to insure against that one entire switch creating so-called dead short that otherwise might preclude proper operation of the remaining ones of the switches in that group and their associated light device.

FIG. 3c is an enlarged representation of a crosssection taken along the line x-x in FIG. 3a and reveals in more detail a form of construction of a switch 30 that is generally similar to but which constitutes an improvement over the kind of switch employed in the aforementioned co-pending application. In this case, each switch includes a plurality of granules or particles of a powder 32 that includes an active amorphous semiconductor material. The powder is sandwiched between a pair of refractory-metal or carbon conductive electrodes 33 and 34 with a further resistive layer 35 disposed on top of electrode 33. All of these elements 32-35 are disposed within a corresponding one of the apertures in sheet 26. The combination of powder 32 and its immediately-adjacent electrodes 33 and 34 constitute the total field-responsive switch. Resistive layer 35 is included as additional insurance against a shorting out of the total local display device in the event of a permanent break-down of an entire individual one of the switches; Moreover, the resistance in series with the electric field path tends to compensate for changes in the electric field pattern that otherwise might occur as each switch becomes conductive.

An alternative and more simplified version of an overall individual switch construction is shown in FIG. 3d. In this case, an insulating layer 24d is disposed only in the region immediately beneath column electrode 21 and the above-discussed anisotropically resistive layer is omitted as in the individual resistive layer associated in FIG. 30 with each of the individual switches. Moreover, each of the latter is in itself simplified by including only the active powder 32 sandwiched between electrodes 33 and 34, theseelements again in the case of each switch 30 being disposed within an aperture through insulating sheet 26d. FIGS. 30 and 3d are intended only to demonstrate by comparison the difference between the comparatively more complex assembly of FIG. 30 and the simpler construction represented in FIG. 3d. Depending upon the degree of tailoring of the shape of the non-uniformly distributed field desired and the degree of protection to be sought against total shorts, any one or more of the additional features of 8 FIGS. 30 may be incorporated into the version of FIG. 3d.

As indicated, the materials interposed between electrode 21 and switches 30 may be manipulated in order to control the transfer curve of light output as a function of control signal. Moreover, care must be taken to insure that a potential applied only to electrode 21, in the absence of a counter-potential applied to electrode 22, cannot result by itself in activation of an associated switch. To this end, an insulated field-shaping conductor may be interposed between column electrode 21 and switches 30. With any given combination of different materials, dielectric constants and dimensions, field mapping is necessary in order to select the final arrangement. Moreover, any one layer or film may be contoured or shaped so as to adjust its effect on the resulting field-spread pattern.

In an alternative modification, the relative positions of the row and column electrodes may be, in one sense, reversed. That is, a column electrode in this case is oriented to run in the same direction as the direction in which switches 30 are dispersed. On the opposite side of the panel, a row electrode is then run in the orthogonal direction. In this case, therefore, the resulting electric field is concentrated at the row electrode and spreads out in the direction in which switches 30 are distributed or dispersed. The field lines thus are distributed more or less in a manner which would be obtained by inverting the field lines and the positions of electrodes 21 and 22 in FIG. 3b. One result of such an arrangement, however, is that the field is more unevenly distributed in its passage through electroluminescent slab 27. In general, this tends to be undesirable with reference to the subsequent application of the sustain signal used as described above to maintain the development of light following the addressing and initial energization of each different display device corresponding to a particular picture element. Accordingly, the subsequent sustain signal may be applied between a column electrode, on the one hand, and a pair of additional electrodes affixed to substrate 28 and spaced respectively on opposite sides of the row electrode. Alternatively the sustain potential or signal may be applied to an additional electrode which overlies the row electrode and is insulated therefrom. In either case, the sustain potential is applied in a manner so as to create a reasonably uniform field distribution, while the switchactuating field is applied in a manner so as to be nonuniform and thus embrace at any instant a percentageof the total number of switches which is proportional to an applied video level.

As thus far described, switches 30 have been visualized as including a separate medium of amorphous semiconducting material respectively associated with each different energizing path. In accordance with one improved alternative, a unitary layer of the material serves as the active element in each of a plurality of switch paths associated with a single display device that occupies one picture-element position. As shown in FIG. 4, an amorphous-semi-conductor layer 74 has a continuous row electrode 75 affixed to one of its major surfaces and a plurality of isolated or mutually-spaced electrodes 76 spaced apart on its other major surface. Each isolated electrode serves in the presence of an applied field together with a portion of slab 74 to define an effectively separate ovonic switch.

In the further alternative of FIG. 5, a random array of conductive droplets 77 are disposed on one major surface of slab 74, while a similar array of conductive droplets 78 are also randomly disposed on its other major surface. A substantial proportion of the different randomly distributed droplets are aligned in opposing pairs in the presence of an electric field so as to cooperate with the intervening portion of slab 74 in each case to form a corresponding plurality of separate switches. In either case, the resulting plurality of switches are substituted in a construction like that of FIG. 3a for sheet 26 and its included switches 30.

Heretofore, amorphous semiconductor switches have usually been formed, as indicated above, by depositing individual electrodes on a sphere or other form of the amorphous-semiconducting material. The size of the semiconductor body has to be sufficient so as to support, in turn, electrodes large enough to permit affixation of connecting leads. In preference thereto, and as mentioned in connection with FIGS. 3c and 3d, the ovonic-type switch material is fabricated in powder form. To this end as shown in FIG. 6, a particle, preferably a sphere 80, of a conductive material is coated with a thin layer 81 of the amorphous semiconducting glass. The material of which sphere 80 is formed may be graphite or a refractory metal such as molybdenum. Suitable amorphous glasses are described in the previously mentioned references. Moreover, it is shown, as such that the amorphous material may exhibit memory as well as threshold operation. Sphere 80 may be a few microns in diameter, while coating 81 may be of the order of one micron thick. The coating may be accomplished by passing the spheres through the amorphous material while the latter is in vapor form.

A complete ovonic switch 30a, that preferably is used in each pore within sheet 26 (FIG. 3a), is depicted in FIG. 7. In this case, each pore is filled with a powder composed of a plurality of the coated spheres of FIG. 6 and sandwiched between a pair of conductive electrodes 82 and 83 that desirably are of carbon or a refractory metal. As between electrodes 82 and 83 in the presence of an electric field, the resulting overall switch system is constituted of a large plurality of individual switches, the individual coated spheres, which are connected in various series and parallel combinations by meansof the totality of metal-glass and glassglass interfaces. The resultant multiplicity of paths between electrodes 82 and 83 results in a built-in redundancy. In turn, such redundancy insures against failure in a completed panel of a portion of its operation as otherwise might occur with failure of but a single switch element at any one location.

To obtain increased stability in the resulting device, it usually is preferred to form the powder after it is situated in place between its associated electrodes. To this end, the switch assembly may be subjected to the application of heat and sometimes also pressure. Altematively, electrical pulses may be applied. In either case, the result is to stabilize the many glass-to-glass and glass-to-metal interfaces.

FIG. 8 depicts a further alternative in which a plurality of the coated spheres 93 are substituted in an arrangement otherwise somewhat similar to that of FIG. 3d. Thus, the spheres are distributed as a powder throughout a space beneath an insulating layer 94 and a glass substrate 95, the composite structure of elements 93-95 being sandwiched between column electrode 21 and electroluminescent slab 27. In addition, a plurality of conductive droplets are deposited on and fixed to the opposite surfaces of layer 94 and substrate 95. Again, the droplets preferably are of a refractory metal. As in FIG. 5, the randomly-distributed droplets find alignment of a sufficient number of opposing pairs as once again to constitute a plurality of dispersed switch elements within each switch system.

In operation, the resulting multiplicity of paths through the powder composed of spheres 93 are dispersed relative to electrode 21 and thus relative to different portions of the ultimate distribution of the resulting non-uniform electric field developed between the column and row conductors. In this way, a plurality of different effectively-separate switch paths are associated with respective different portions of the electroluminescent material so that the overall operation is generally the same as that described in connection with FIGS. 30 and 312. Moreover, the layer of spheres may extend entirely across the panel so as to constitute a plurality of groups each associated with a separate picture element.

The various different alternatives and modifications afford substantial selectivity in the manner of final approach to construction as well as permitting the inclusion of one or more of such features as redundancy and simplicity of manufacture. At the same time, the actual fabrication techniques demanded require essentially no greater level of technical competence than that finding present-day use in connection with the formation of integrated circuits, printed components, thin-film devices and the like. Whether used in a display panel or in some other application, the powdered form of the switches may in themselves be fabricated by use of conventional techniques. For example, the powder may be deposited from a shaker or a spout, or it may be applied by spraying, painting or silk-screening.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made therein without departing from the invention in its broader aspects. The aim of the appended claims, therefore, is to cover all changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A threshold switch system comprising:

a plurality of spaced conductive electrodes;

and a powder, exhibiting a change in electric conductivity in response to an applied field, sandwiched between said electrodes and including a plurality of minute particles of conductive material with each of said particles being individually surrounded by a coating of amorphous semiconducting material.

2. A switch system as defined in claim 1 in which said electrodes are in the form of a pair of spaced continu: ous layers.

3. A switch system as defined in claim 1 in which said electrodes include at least one continuous conductive layer disposed on one side of said powder.

4. A switch system as defined in claim 1 in which said electrodes include at least one group of mutuallyspaced electrode elements disposed on one side of said powder.

5. A switch system as defined in claim 4 in which said electrode elements are randomly distributed over said powder.

6. A switch system as defined in claim 4 which further includes a sheet of electrically insulating material, and said electrode elements are affixed to said sheet and sandwiched between the latter and said powder.

7. A switch system as defined in claim 1 in which said electrodes include a pair of groups of mutually-spaced electrode elements disposed on respective opposite sides of said powder.

8. A switch system as defined in claim 1 in which said powder is disposed in a continuous layer and in which said electrodes include at least one group of mutuallyspaced electrode elements disposed on one side of said continuous layer.

stabilization of the plurality of different interfaces. 

2. A switch system as defined in claim 1 in which said electrodes are in the form of a pair of spaced continuous layers.
 3. A switch system as defined in claim 1 in which said electrodes include at least one continuous conductive layer disposed on one side of said powder.
 4. A switch system as defined in claim 1 in which said electrodes include at least one group of mutually-spaced electrode elements disposed on one side of said powder.
 5. A switch system as defined in claim 4 in which said electrode elements are randomly distributed over said powder.
 6. A switch system as defined in claim 4 which further includes a sheet of electrically insulating material, and said electrode elements are affixed to said sheet and sandwiched between the latter and said powder.
 7. A switch system as defined in claim 1 in which said electrodes include a pair of groups of mutually-spaced electrode elements disposed on respective opposite sides of said powder.
 8. A switch system as defined in claim 1 in which said powder is disposed in a continuous layer and in which said electrodes include at least one group of mutually-spaced electrode elements disposed on one side of said continuous layer.
 9. A switch system as defined in claim 1 in which said powder is distributed into a group of mutually-spaced cells all sandwiched between said electrodes.
 10. A switch system as defined in claim 1 which further includes a sheet of electrically insulating material; means defining a plurality of apertures in said sheet; and a plurality of portions of said powder individually disposed in respective different ones of said apertures.
 11. A switch system as defined in claim 1 in which the coated particles of said powder are formed together in stabilization of the plurality of different interfaces. 